bigquery: make FieldValue publicly construct-able

[pongad]

so make testing easier for users. Fixes #2880.

[pongad]

@vam-google Are you OK with this being public?

[tswast]

Hmm, I think something like #1370 would better cover the desired use case for testing specific query / table values than exposing this as public would. Albeit, that is a bigger project than changing visibility.

[pongad]

I agree; doing that would definitely be better. I guess there are 2 options.

1. We can go ahead with this PR, and mark the newly-public methods @BetaApi. When POJO works, we deprecate them.
2. We close FieldValue.of is not public #2880 as dup of bigquery: support loading a POJO or Map like GSON does #1370, which is now tracked by feature wiki.

I'll defer to you on the decision. However, I'm slightly leaning towards (1). My reasoning is that, right now, users have to deal FieldValueList.get anyway. If you know what to expect from get, calling of isn't too terrible.

[tswast]

Yeah, (1) makes sense. I agree to keep it as @BetaApi for now.

[tswast]

LGTM, assuming you add @BetaApi.